Come valuta `cos ((2pi) / 3)`?

The angle `(2pi)/3" is in the 2nd quadrant "`
where the cos ratio has a negative value.

The related 'acute angle' to `(2pi)/3 =( pi -(2pi)/3) = pi/3`

`rArr cos((2pi)/3) = - cos(pi/3)`

and using the `color(blue)" Exact value triangle"`

`cos((2pi)/3) = - cos(pi/3) = -1/2`